Why does the hashCode() of an ArrayList change every time you add a new element?

Question

As per my understanding of ArrayList, the default capacity is 10 and when it grows beyond 10, it will create a new object with new capacity and so on..

So out of curiosity, I typed following program to check hashcode() for ArrayList object:

public class TestCoreJava {      public static void main(String [] args){          ArrayList al = new ArrayList();          for(int i=0;i<15;i++){              al.add("Temp"+i);             System.out.println("Hashcode for "+i+" element "+al.hashCode());         }     } } 

According to above scenario, when I am not setting Initial capacity for ArrayList the default would be 10. So while adding 11th element, it will create a new object and increase the capacity for ArrayList.

When I print the hashcode for ArrayList object, it is giving a new hashcode() each time.

Following is the o/p:

Hashcode for 0 element 80692955 Hashcode for 1 element -1712792766 Hashcode for 2 element -1476275268 Hashcode for 3 element 1560799875 Hashcode for 4 element 1220848797 Hashcode for 5 element -727700028 Hashcode for 6 element -1003171458 Hashcode for 7 element -952851195 Hashcode for 8 element 607076959 Hashcode for 9 element 1720209478 Hashcode for 10 element -6600307 Hashcode for 11 element -1998096089 Hashcode for 12 element 690044110 Hashcode for 13 element -1876955640 Hashcode for 14 element 150430735 

According to the concept of default capacity, till 10th element it should have printed same hashcode() as no new object needs to be created until that point, but it is not the case.

Answer 1

The hashCode of ArrayList is a function of the hashCodes of all the elements stored in the ArrayList, so it doesn't change when the capacity changes, it changes whenever you add or remove an element or mutate one of the elements in a way that changes its hashCode.

Here's the Java 8 implementation (it's actually implemented in AbstractList) :

public int hashCode() {     int hashCode = 1;     for (E e : this)         hashCode = 31*hashCode + (e==null ? 0 : e.hashCode());     return hashCode; } 

BTW, this is the exact code that appears in the Javadoc of hashCode() of the List interface :

int java.util.List.hashCode()

Returns the hash code value for this list. The hash code of a list is defined to be the result of the following calculation:

int hashCode = 1; for (E e : list)     hashCode = 31*hashCode + (e==null ? 0 : e.hashCode());